1. Field of the Invention
The present invention is related to metal-semiconductor junction devices, and more particularly to devices based on metal-semiconductor junctions such as Schottky diodes, photo-diodes, MESFETs, etc., produced from metal oxides such as ZnO.
2. Description of the Prior Art
Due to its unique material properties, zinc oxide (ZnO) has been and continues to be used in optoelectronic components. For example, ZnO is a semiconductor with a direct band gap of 3.37 eV (368 nm at room temperature). Its transparency for the visible spectrum and conductivity mean that ZnO can be used as a transparent electrode, for example in optoelectronic applications such as light emitting diodes (LEDs), laser diodes, photodiodes, optical displays, etc.
While ZnO may serve as a suitable bulk material for device fabrication, doping is required to actually realize typical semiconductor devices. ZnO can be doped n-type, for example by introduction of Ga or Al as when the material is used as transparent conducting oxide. However, reliable p-doping in ZnO has yet to be demonstrated. Consequently, it has not been possible to form p-junction or p-n junction devices, such as diodes and diode-based devices like LEDs, laser diodes or photodiodes. Also for electronic devices such as bipolar transistors and junction FETs p doping is required.
For certain applications, Schottky diodes (metal-semiconductor junction devices) are used as an alternative to semiconductor-semiconductor junction devices. In addition, Schottky devices enable a number of unique applications, e.g. MESFETs, Schottky photodiodes, etc. However due to the low absolute energy of the conduction and valence band of bulk ZnO, the fabrication of high quality Schottky contacts of ZnO is problematic. Many unconventional fabrication methods have been proposed, however none of them provides a reliable, reproducible and convenient method to fabricate Schottky contacts.
There is little information about ZnO-metal Schottky contacts in the open literature to date. The chemical reactions between the metal and the semiconductor, the surface states, the contaminants, the defects in the surface layer, and the diffusion of the metal into the semiconductor are well known problems in the formation of Schottky contacts. For instance, with Al as the contact metal, ZnO produces significant dissociated cations (Zn) in ZnO because of its strong reaction with anions (O) in ZnO. This results in low barrier height and high leakage current.
To create a Schottky barrier with undoped ZnO, a high work function metal can be applied to the surface of a ZnO crystal. Although it has been shown that Au presents a number of challenges at high temperatures (>340 K), Au has widely been applied to ZnO to form Schottky barriers. Other metals used for the same purpose are Ag and Pd. It has been found that all these reactive metals form relatively high Schottky barriers of 0.6-0.84 eV to the n-type ZnO.
Since high-quality Schottky contacts on ZnO are problematic and reliable p-doping of ZnO has not previously been demonstrated, there has been little work addressing the use of ZnO for UV photodetection, including photoconductors, Schottky barrier photodetectors, metal-semiconductor-metal (MSM) structures, etc. ZnO photoconductors, consisting of two ohmic Al contacts on N-doped ZnO grown by metal-organic vapor phase epitaxy (MOVPE) have been reported. (“Ultraviolet detectors based on Epitaxial ZnO films grown by MOCVD,” TMS & IEEE J. Electronic Materials, 27, 69-74 (January 2000), incorporated herein by reference.) At +5 V bias, these devices present a dark current of 450 nA, a responsivity of ˜400 NW and a time response of 1.5 μs. In a similar material, MSM photodiodes formed with interdigitated Ag Schottky contacts present lower leakage current (1 nA at 5 V bias) and better spectral selectivity, but slower time response (“ZnO Schottky ultraviolet photodetectors,” J. Crystal Growth, vol. 225, pp. 110-113 (May 2001), which is incorporated herein by reference). The fast response characteristic of this structure is followed by a slow photocurrent decay, which lasts for about 5 ms. This slow component is attributed to the oxygen adsorption at the surface and grain boundaries.
In an effort to overcome the difficulty forming p-n junction devices, another approach is to use a second semiconductor material on the p-side of the device while using ZnO on the n side. Heterojunction diodes have been produced in this way, but these devices exhibit substantial material quality issues due to the lattice and thermal mismatch during growth. Nevertheless, efforts continue with regard to studying and developing the photo response properties of these ZnO based heterojunctions.
For example, Jeong et al. in “Ultraviolet-enhanced photodiode employing n-ZnO/p-Si structure,” Appl. Phys. Lett. 83, 2946 (2003), which is incorporated herein by reference, reported on the photoelectric properties of a heterostructure n-ZnO/p-Si photodiode which detect UV photons in the depleted n-ZnO and simultaneously detects visible photons in the depleted p-Si by employing two related photoelectric mechanisms. The I-V measurements obtained while the photodiodes are exposed to radiation in a wavelength range of 310 to 650 nm showed a linear increase in photocurrent with reverse bias. In the visible range, the photocurrent rose rapidly with bias but saturated beyond a critical voltage. According to this reference, the diodes exhibited high responsivities of 0.5 and 0.3 NW for UV (310-nm) and red (650-nm) photons, respectively, under a 30 V bias with a minimum near 380 nm which corresponds to the band gap of ZnO.
However, there remain both a desire and a need in the art for techniques for producing a high quality Schottky contact device based on ZnO (or more generally in metal oxides), particularly one with low leakage current and desired optical and I-V properties.